1. Field of the Invention
This invention pertains to a novel slurry concentrator and methods of use. The apparatus is particularly useful in concentrating an aqueous proppant slurry for hydraulic fracturing of a subterranean earth formation.
2. Description of the Prior Art
A variety of methods have been described in the prior art to stimulate the production of hydrocarbon fluids (e.g. crude oil and natural gas) from wells drilled into reservoirs of low permeability. Matrix acidizing and hydraulic fracturing of such formations are the best known and most widely used techniques. In hydraulic fracturing, a fracturing fluid is pumped through a perforated casing into the hydrocarbon-bearing pay zone under pressure sufficiently high to cause the formation to split or fracture. The fracture fluid normally contains a proppant which is emplaced into the fracture and prevents the formation from closing back upon itself when the pumping pressure is released. The propping agent (e.g. sand, sintered bauxite, glass beads, etc.) provides a permeable channel in the formation through which fluids flow to the well bore and are withdrawn.
In the usual hydraulic fracturing process, various liquids such as crude oil, diesel fuel, kerosene, water, gelled water, acid (aqueous HCl), gelled acid, etc. (with or without propping agents) have been used as the fracturing fluid. Water gelled with a polysaccharide or polysaccharide derivative (e.g. guar, hydroxypropyl guar, etc.) has been a particularly effective fracturing fluid and has been widely used. However, these aqueous gels and other liquid fracturing fluids have problems associated with their use. For example, when the formation breakdown pressure is reached and fracturing occurs, a large pressure drop results. The pumps must then be capable of supplying large volumes of additional fracturing liquid quickly to the fracturing zone in order to maintin and extend the fractures formed. To supply such additional pump pressure requires several large pumping units to be held in reserve which would not otherwise be required. In addition, liquid fracturing fluids often require additional treatment or chemical additives to increase their viscosity, gel strength, and/or to improve their fluid loss properties during use. Further treatment of the liquids have been designed to improve the proppant carrying capabilities of the fluids.
Many of the problems associated with liquid fracturing fluids have been addressed and solved by hydraulic fracturing with foam. Foam fracturing is of relatively recent vintage and the state of the art is represented by Blauer, et al. in U.S. Pat. No. 3,937,283. Other articles of interest in foam fracturing appear in the July 22, 1974 issue of "Oilweek" by Bullen and in the paper presented by Blauer, et al. entitled "Formation Fracturing with Foam" SPE 5003. Reference is also made to Plummer et al., U.S. Pat. No. 3,980,136.
Blauer et al. in U.S. Pat. No. 3,937,283 describe the many advantages of foam fracturing and teach methods of making and using such foams. One of the advantages of foam fracturing is lower fluid loss to the formation. Lower fluid loss means that the fracture treatment is more efficient and that formation damage is minimized. As a further advantage, the foams typically have a relatively high proppant carrying capacity which allows more of the proppant to be delivered to the site of the fracture and this helps eliminate screen-outs during unplanned shut-downs during the fracturing treatment. A "screen-out" is, of course, a term used in the oilfield to define the phenomenon when the proppant precipitates out of the fracturing fluid and plugs up the wellbore. The foams are also easier to pump and require less initial hydraulic horsepower and fewer reserve pumping units at the well site.
Foam fracturing efficiency can be further enhanced by concentrating the proppant slurry before generating the foam. The proppant slurry comprises a foamable liquid and particulate solid proppant. Presently, conventional proppant slurries contain concentrations of from about 6 to about 8 pounds of proppant (e.g. sand) per gallon of carrier fluid; after foaming, the solids content of the slurry is reduced to about 1 or 2 pounds of proppant per gallon of liquid. Since it is at least theoretically possible for the foam to support substantially more proppant than this, a way is needed to increase the slurry density prior to foaming.
Black described one device in U.S. Pat. Nos. 4,176,064 and 4,126,181 to accomplish this result. The slurry concentrator described by Black is a mechanical separator having a rotating impeller that imparts a centrifical force to an incoming slurry which forces the heavier materials to the outside walls and lets the lighter carrier fluid separate from the slurry and flow through a slotted screen out of the concentrator into a reservoir. A commercial embodiment of this concentrator has enjoyed only marginal success due to mechanical problems associated with the rotating parts, seals, etc. In actual practice, such problems have resulted in the operating pressure of the units being usually restricted to a maximum of about 5000 psi (35,000 kPa) and moderate volume throughput. Further information on this system can be found in the paper presented by Anderson et al. (Paper No. 793,039) presented at the 30th annual technical meeting of the Petroleum Society of CIM on May 8-11, 1979, in Banff.
Sales literature by a variety of oil well service companies indicates that other slurry concentrators are being developed for use in foam fracturing.
The disclosures of each of the patents and journal articles referred to above are specifically incorporated herein by reference.